Reduction of ion thermal transport due to non-uniformity effects of the radial electric field at the edge of the JT-60U tokamak

摘要

Non-uniformity effects of the edge radial electric field, E_r, on edge transport barrier (ETB) formation have been identified with high-spatiotemporal resolution spectroscopic measurement. We identified the decisive importance of E_r-curvature (i.e. 2nd derivative of E_r) on ETB formation corresponding to a local peak value in the pedestal region of the normalized ion temperature gradient, L_(Ti)~(-1) = —▽T_i/T_i. On the other hand, the E_r-shear (i.e. 1st derivative of E_r) was discovered to promote the expansion of pedestal width, compensating an unfavorable effect of the fir-curvature having its sign dependence on the transport reduction/enhancement—predicted by a theoretical model as L_(Ti)~(-1) α(E'_rE'_r — E_rE"_r)~(0.5). A significant reduction of the inferred ion thermal transport coefficient in the pedestal region, X_(i,ped.), due to the non-uniformity effects of E_r was also confirmed quantitatively in the H-mode plasma at the initial ELM-free phase. When the inhomogeneity of radial electric field became strong enough, the inferred X_(i,ped). value was close to its neoclassical one at the later ELM-free H-phase until 1st type-I ELM onset. Nevertheless, we observed the further reduction of electron density fluctuations (in the range up to 300 kHz) and D_a signals in association with the increment of electric field inhomogeneity. These suggest that the electron turbulent transport is suppressed further.